Technical Field
[0001] The present invention relates to a high-tensile steel plate used for steel structures
such as ships, marine structures, pressure vessels, and penstocks and to a method
for producing the high-tensile steel plate. In particular, the present invention relates
to a high-tensile steel plate having a yield point of 620 MPa or more and realizing
high low-temperature toughness of a multipass welded zone formed by low-to-medium
heat input welding as well as high base-material strength and toughness and to a method
for producing the high-tensile steel plate.
Background Art
[0002] Steel used for ships, marine structures, and pressure vessels is formed into a structure
having a desired shape by weld bonding. Therefore, it is required that such steel
realize high toughness of welded joint portions (e.g., weld metal and heat affected
zone) as well as, needless to say, high base-material strength and toughness from
the viewpoint of safety of structures.
[0003] Hitherto, absorbed energy determined by a Charpy impact test has been mainly used
as a standard for evaluating the toughness of steel. In order to enhance reliability,
recently, a crack tip opening displacement test (hereinafter, referred to as "CTOD
test") has been commonly employed. A CTOD test evaluates resistance to brittle fracture
by performing three-point bending of a test piece having a fatigue crack formed in
a toughness evaluation portion and then measuring an opening displacement at the crack
tip immediately prior to fracturing.
[0004] Since a CTOD test utilizes a fatigue crack, a very minute region serves as a toughness
evaluation portion. Thus, if a local brittle zone is present, low toughness may be
measured by a CTOD test even though high toughness is measured by a Charpy impact
test.
[0005] A local brittle zone is likely to be formed in a weld heat affected zone (HAZ) that
is subjected to a complex thermal history due to multipass welding of a thick steel
plate or the like. A bonded portion (interface between a weld metal and a base material)
and a portion in which the bonded portion is reheated to form a dual-phase region
(portion in which coarse particles are formed in the first weld cycle and a dual-phase
region of ferrite and austenite is formed due to heating by the following weld path,
hereinafter, referred to as "dual-phase-region reheated portion") may become a local
brittle zone.
[0006] A bonded portion is subjected to a high temperature near its melting point, which
increases the size of austenite grains, and is likely to be caused to be transformed
into an upper bainite structure having low toughness by the subsequent cooling. Therefore,
the matrix itself has low toughness. In addition, brittle structures such as a Widmannstatten
structure and a martensite-austenite constituent are likely to be formed in a bonded
portion, which causes further degradation of toughness.
[0007] In order to enhance the toughness of a bonded portion, for example, a technique in
which TiN is finely dispersed in steel and thereby an increase in the size of austenite
grains is suppressed or the dispersed TiN is utilized as ferrite transformation cores
has been in practical use.
[0008] Patent Literatures 1 and 2 disclose a technique of enhancing welded portion toughness
by adding a rare-earth metal (REM) to steel in combination with Ti, dispersing fine
particles in the steel, and thereby suppressing growth of austenite grains.
[0009] In addition, a technique in which an oxide of Ti is dispersed, a technique in which
ferrite-core formation capacity of BN is utilized in combination with dispersing of
an oxide, and a technique of enhancing toughness by adding Ca and REM and thereby
controlling the form of a sulfide are described.
[0010] Patent Literature 3 proposes a V-free refined high-tensile steel because, in the
case of multipass welding, a brittle zone due to precipitation hardening of V, which
is a precipitation-type element, serves as a local brittle zone in a CTOD test and
this reduces a critical CTOD value.
[0011] However, the above techniques are intended for steel materials having a relatively
low strength and less amounts of alloy elements and are not applicable to steel materials
having a high-tensile and large amounts of alloy elements because, in this case, a
HAZ microstructure does not include ferrite.
[0012] Patent Literature 4 discloses a technique for promoting formation of ferrite in a
weld heat affected zone mainly by increasing the amount of Mn added to 2% or more.
Patent Literature 5 describes a technique for improving CTOD characteristics (CTOD
toughness) of a HAZ by making the microstructure of a weld heat affected zone finer
by employing a high-Mn type chemical composition, controlling the amount of oxygen
to an appropriate value, and thereby increasing the number of intra-granular transformation
ferrite cores as well as by controlling a value of a parametric expression consisting
of brittle elements such as C, Nb, and V.
[0013] However, alloy elements such as Mn are likely to segregate at the center of a slab
in a continuous-cast material. This increases the hardness of a center-segregation
zone in a weld heat affected zone as well as in a base material and the center-segregation
zone becomes a starting point of fracturing. As a result, base-material toughness
and HAZ toughness become degraded.
[0014] Patent Literature 6 proposes a technique in which a strand having no center segregation
is produced by reducing the thickness of the strand by pressing the strand with a
plane during solidification subsequent to continuous casting and a microstructure
in the vicinity of a weld bonded portion is improved using a complex oxide.
[0015] Patent Literature 7 proposes a technique of designing components by determining an
average analytical value of the components contained in a microscopic region including
segregation of the central portion in a plate-thickness direction located at a position
corresponding to the center of a slab and thereby deriving a segregation parametric
expression.
[0016] In a dual-phase-region reheated portion, carbon concentrates at a region that has
reverse-transformed into austenite due to dual-phase-region reheating and thereby
a vulnerable bainite structure including a martensite-austenite constituent is formed
during cooling, which causes degradation of toughness. Patent Literatures 8 and 9
disclose a technique in which toughness is improved by setting a steel chemical composition
to contain low C and low Si and thereby suppressing formation of a martensite-austenite
constituent and base metal strength is maintained by adding Cu. In the above technique,
strength is increased by precipitation of Cu through an aging treatment, and a large
amount of Cu is added. This causes degradation of hot ductility and accordingly deteriorates
productivity.
[0017] As described above, various factors affect CTOD characteristics. Thus, Patent Literature
10 proposes a steel material with which good CTOD characteristics of a multipass welded
zone formed by low-to-medium heat input welding are realized. The steel material is
produced by taking comprehensive measures such as control of slab-heating temperature
for a continuous casting steel slab such that center segregation is reduced, control
of the amount of B mixed into a steel chemical composition, and control of a chemical
composition with which formation of a martensite-austenite constituent is suppressed.
[0018] Patent Literature 11 describes a technique for improving CTOD characteristics of
a multipass welded zone formed with a welding heat input up to 100 kJ/cm at maximum
by, in the case of large-heat input welding, making effective crystal grains that
are units into which HAZ coarse grains are broken finer and, in the case of low-to-medium
heat input welding, setting a chemical composition capable of improving grain boundary
hardenability due to a reduction in the amount of a martensite-austenite constituent
and addition of a trace amount of Nb, suppressing of precipitation hardening, and
reducing the hardness of a HAZ.
[0019] JP H08176724 discloses a high tensile steel having excellent welding cold crack resistance and
the production method thereof.
Citation List
Patent Literature
[0020]
PTL 1: Japanese Examined Patent Application Publication No. 3-053367
PTL 2: Japanese Unexamined Patent Application Publication No. 60-184663
PTL 3: Japanese Unexamined Patent Application Publication No. 57-9854
PTL 4: Japanese Unexamined Patent Application Publication No. 2003-147484
PTL 5: Japanese Unexamined Patent Application Publication No. 2008-169429
PTL 6: Japanese Unexamined Patent Application Publication No. 9-1303
PTL 7: Japanese Unexamined Patent Application Publication No. 62-93346
PTL 8: Japanese Unexamined Patent Application Publication No. 5-186823
PTL 9: Japanese Unexamined Patent Application Publication No. 2001-335884
PTL 10: Japanese Unexamined Patent Application Publication No. 2001-11566
PTL 11: Japanese Unexamined Patent Application Publication No. 11-229077
Summary of Invention
Technical Problem
[0021] In a jack-up rig used in recent marine structures, a steel material having a yield
point of 620 MPa class and a plate thickness of 50 to 210 mm is used as a leg portion,
a cantilever (beam of a drill portion), and the like. Therefore, good CTOD characteristics
in a welded portion are requested. However, it is difficult to employ the techniques
for improving the CTOD characteristics of a weld heat affected zone described in Patent
Literatures 1 to 11 because the target yield point and/or plate thickness of a steel
material are different.
[0022] Accordingly, an object of the present invention is to provide a high-tensile steel
plate having a yield point of 620 MPa or more and realizing good CTOD characteristics
of a weld heat affected zone in a multipass welded zone formed by low-to-medium heat
input welding, which is suitably used for steel structures such as ships, marine structures,
pressure vessels, and penstocks, and to provide a method for producing the high-tensile
steel plate.
Solution to Problem
[0023] The inventors of the present invention have conducted extensive studies on a method
for improving the toughness of a weld heat affected zone formed by multipass welding
in order to maintain CTOD characteristics, that is, a critical CTOD value of 0.50
mm or more at a test temperature of -10°C as well as maintaining base-material strength,
that is, a yield point of 620 MPa or more, and base-material toughness.
[0024] As a result, the inventors have found the following effective methods: 1. suppressing
an increase in the size of austenite grains in a weld heat affected zone; 2. dispersing
transformation cores uniformly and finely in order to promote ferrite transformation
upon cooling subsequent to welding; 3. controlling the amount of Ca, which is added
in order to control the form of a sulfide, within an appropriate range in order to
suppress formation of a brittle structure; and 4. controlling the contents of C, P,
Mn, Nb, and Mo, which are brittle elements, within a appropriate range in order to
improve the CTOD characteristics of a weld heat affected zone.
[0025] The present invention has been made by conducting further studies on the basis of
the above findings and established the scope according to the claims.
[0026] According to the present invention, a high-tensile steel plate having a yield point
of 620 MPa or more and realizing high low-temperature toughness, in particular, good
CTOD characteristics, of a multipass welded zone formed by low-to-medium heat input
welding, which is suitably used for large steel structures such as a marine structure,
and a method for producing the high-tensile steel plate can be produced and are very
useful industrially.
Description of Embodiments
[0027] In the present invention, chemical composition and hardness distribution in a plate-thickness
direction are specified.
1. Chemical composition
[0028] The reasons for the limitations on the chemical composition are described. In the
following description, "%" represents "mass%".
C: 0.05% to 0.14%
[0029] C is an element that is necessary in order to maintain base-material strength for
a high-tensile steel plate. If the C content is less than 0.05%, hardenability becomes
degraded, which requires addition of large amounts of elements that enhance hardenability,
such as Cu, Ni, Cr, and Mo, in order to maintain strength. This leads to a high cost
and degradation of weldability. On the other hand, if the amount of C added exceeds
0.14%, weldability becomes significantly degraded and the toughness of a welded zone
becomes degraded. Thus, the C content is set to 0.05% to 0.14% and preferably set
to 0.07% to 0.13%.
Si: 0.01% to 0.30%
[0030] Si is a component that serves as a deoxidizing element and that is added in order
to maintain base-material strength. However, a large amount of Si exceeding 0.30%
results in degradation of weldability and degradation of the toughness of a welded
joint. Thus, it is necessary to set the Si content to 0.01% to 0.30%. Preferably,
the Si content is 0.25% or less.
Mn: 0.3% to 2.3%
[0031] The amount of Mn added is 0.3% or more in order to maintain base-material strength
and the strength of a welded joint. If the amount of Mn added exceeds 2.3%, weldability
becomes degraded and hardenability becomes excessively enhanced, which results in
degradation of base-material toughness and the toughness of a welded joint. Thus,
the Mn content is set to 0.3% to 2.3%.
P: 0.008% or less
[0032] P is an impurity that is inevitably mixed into steel and causes base-material toughness
and the toughness of a welded zone to be degraded. In particular, if the P content
exceeds 0.008% in a welded zone, toughness becomes significantly degraded. Thus, the
P content is set to 0.008% or less.
S: 0.005% or less
[0033] S is an impurity that is inevitably mixed into steel. If the S content exceeds 0.005%,
base-material toughness and the toughness of a welded zone become degraded. Thus,
the S content is set to 0.005% or less and preferably set to 0.0035% or less.
Al: 0.005% to 0.1%
[0034] Al is an element that is added in order to deoxidize molten steel, and it is necessary
to set the Al content to 0.005% or more. However, if the amount of Al added exceeds
0.1%, base-material toughness and the toughness of a welded zone become degraded.
Furthermore, Al is diluted due to welding and mixed into a weld metal zone, which
causes toughness to be degraded. Thus, the Al content is limited to 0.1% or less and
preferably limited to 0.08% or less.
Ni: 0.5% to 4%
[0035] Ni causes the strength and toughness of steel to be enhanced and is therefore effective
for enhancing the low-temperature toughness of a welded zone. Thus, the Ni content
is set to 0.5% or more. However, Ni is an expensive element and addition of an excessive
amount of Ni causes hot ductility to be degraded, which increases of the risk of formation
of flaws in the surface of a slab during casting. Thus, the upper limit is set to
4%.
B: 0.0003% to 0.003%
[0036] B segregates at the austenite grain boundary and suppresses the ferrite transformation
starting from the grain boundary. Thus, addition of a trace amount of B produces an
effect of enhancing the hardenability of steel. This effect is produced when the amount
of B added is 0.0003% or more. However, if the B content exceeds 0.003%, B precipitates
as a carbonitride or the like, which reduces hardenability and toughness. Thus, the
B content is set to 0.0003% to 0.003% and preferably set 0.0005% to 0.002%.
N: 0.001 to 0.008%
[0037] N reacts with Al and thereby forms a precipitate. This makes crystal grains finer,
which enhances base-material toughness. N is an element that is necessary for forming
TiN, which suppresses an excessive increase in the size of the microstructure of a
welded zone. Thus, the N content is set to 0.001% or more. However, if the N content
exceeds 0.008%, base-material toughness and the toughness of a welded zone become
significantly degraded. Thus, the upper limit is set to 0.008%.
[0038] If Ceq exceeds 0.80, weldability and the toughness of a welded zone become degraded.
Thus, Ceq is set to 0.80 or less and is preferably set to 0.75 or less. Note that,
Ceq = [C] + [Mn]/6 + [Cu + Ni]/15 + [Cr + Mo + V]/5. Each symbol of element represents
its content (mass%) and is 0 when the element is not contained.
[0039] HCS = 5.5[C]
4/3 + 15[P] + 0. 90[Mn] + 0.12[Ni] + 0.53[Mo] ≤ 2.5, where [M] represents the content
(mass%) of the element and is 0 when the element is not contained.
[0040] This parametric expression is a center-segregation zone hardness index consisting
of components that are likely to concentrate in a center-segregation zone, which is
obtained empirically. If the value of the parametric expression exceeds 2.5, CTOD
characteristics become degraded. Therefore, the value of the parametric expression
is set to 2.3 or less. Since a CTOD test examines a steel plate over its entire thickness,
a test piece including a center segregation is evaluated in terms of toughness. If
concentration of components due to center segregation is significant, a hardened zone
is formed in a weld heat affected zone, which prevents a good measurement value from
being observed.
[0041] Fundamental chemical compositions of the present invention are described above. In
order to further improve characteristics, one or more elements selected from Cr: 0.2%
to 2.5%, Mo: 0.1% to 0.7%, V: 0.005% to 0.1%, Cu: 0.49% or less, Ti: 0.005% to 0.025%,
and Ca: 0.0005% to 0.003% are added.
Cr: 0.2% to 2.5%
[0042] Cr is an element that is effective for increasing base-material strength when the
amount of Cr added is 0.2% or more. However, addition of an excessive amount of Cr
produces an adverse effect in terms of toughness. Thus, when Cr is added, the Cr content
is set to 0.2% to 2.5%.
Mo: 0.1% to 0.7%
[0043] Mo is an element that is effective for increasing base-material strength when the
amount of Mo added is 0.1% or more. However, addition of an excessive amount of Mo
produces an adverse effect in terms of toughness. Thus, when Mo is added, the Mo content
is set to 0.1% to 0.7% and is preferably 0.1% to 0.6%.
V: 0.005% to 0.1%
[0044] V is an element that is effective for increasing the strength and improving base-material
toughness when the amount of V added is 0.005% or more. However, if the amount of
V added exceeds 0.1%, toughness becomes degraded. Thus, when V is added, the V content
is set to 0.005% to 0.1%.
Cu: 0.49% or less
[0045] Cu is an element having an effect of increasing the strength of steel. However, if
the Cu content exceeds 0.49%, hot embrittlement is caused, which results in degradation
of the surface quality of a steel plate. Thus, when Cu is added, the Cu content is
set to 0.49% or less.
Ti: 0.005% to 0.025%
[0046] Ti precipitates as TiN upon solidification of molten steel, which suppresses an increase
in the size of austenite in a welded zone and thereby contributes to enhancement of
toughness in a welded zone. However, this effect is small if the amount of Ti added
is less than 0.005%. On the other hand, if the amount of Ti added exceeds 0.025%,
the size of TiN excessively increases and it becomes impossible to produce an effect
of improving base-material toughness and the toughness of a welded zone. Thus, when
Ti is added, the Ti content is set to 0.005% to 0.025%.
Ca: 0.0005% to 0.003%
[0047] Ca is an element that fixes S and thereby enhances toughness. In order to produce
this effect, the amount of Ca added needs to be at least 0.0005%. However, if the
Ca content exceeds 0.003%, the effect of Ca becomes saturated. Thus, when Ca is added,
the Ca content is set to 0.0005% to 0.003%.
2. Hardness distribution
[0048] HV
max/HV
ave ≤ 1.35 + 0.006/C - t/750, where C represents carbon content (mass%) and t represents
plate thickness (mm)
[0049] HV
max/HV
ave is a dimensionless parameter that represents the hardness of a center-segregation
zone. If this value exceeds a value calculated by 1.35 + 0.006/C - t/750, the CTOD
value becomes reduced. Thus, HV
max/HV
ave is set to be 1.35 + 0.006/C - t/750 or less.
[0050] HV
max represents the hardness of a center-segregation zone and is determined as the maximum
value among values obtained by measuring a range of (plate thickness/10) mm including
a center-segregation zone at intervals of 0.25 mm in the plate-thickness direction
with a Vickers hardness tester (load: 10 kgf). HV
ave represents an average hardness and is determined as the average of values obtained
by measuring a range that extends from (plate thickness/4) mm below the front side
to (plate thickness/4) below the back side and does not include the center-segregation
zone at intervals of 1 to 2 mm at a load of 10 kgf with a Vickers hardness tester.
[0051] The steel according to the present invention is preferably produced by the method
described below.
[0052] Molten steel having a chemical composition adjusted to be within the range of the
present invention is prepared by an ordinal method using a converter, an electric
furnace, a vacuum melting furnace, or the like and then formed into a slab through
a step of continuous casting. Subsequently, the slab is hot-rolled to a desired plate
thickness, cooled, and then subjected to a tempering treatment.
Slab-heating temperature: 1050°C or more and rolling reduction ratio: 2 or more
[0053] In the present invention, the slab-heating temperature and the rolling reduction
ratio (= slab thickness/plate thickness) during hot rolling have little effect on
the mechanical characteristics of a steel plate. However, in the case of a thick material,
if the slab-heating temperature is too low or rolling reduction amount is insufficiently
small, initial defects caused in production of steel ingot remain in the central portion
of a steel plate in its thickness direction, which causes the internal quality of
the steel plate to be significantly degraded. Thus, the slab-heating temperature is
set to 1050°C or more and the rolling reduction ratio is set to 2 or more in order
to press-bonding such casting defects present in a slab by hot rolling with certainty.
[0054] It is not necessary to set the upper limit for the slab-heating temperature. However,
heating temperature is preferably 1200°C or less because heating at an excessive high
temperature increases the size of a precipitate such as TiN precipitated upon solidification,
which reduces base-material toughness and the toughness of a welded zone and because
a thick scale is formed on the surface of steel slab at a high temperature, which
causes occurrence of surface flaws during rolling. The above heating temperature is
also preferable from the viewpoint of energy conservation.
Cooling after hot-rolling: to 350°C or less at a cooling rate of 0.3°C/s or more
[0055] If cooling rate is less than 0.3°C/s, sufficient base-material strength cannot be
achieved. If cooling is stopped at a temperature higher than 350°C, γ-to-α transformation
cannot be perfectly completed and high-temperature transformation structure is formed.
As a result, high-tensile and high toughness cannot be achieved at the same time.
Cooling rate is measured at the central portion of a steel plate in its thickness
direction. The temperature at the central portion in the plate-thickness direction
can be calculated from plate thickness, surface temperature, cooling conditions, and
the like by simulation calculation or the like. For example, the temperature of the
central portion in the plate-thickness direction is determined by calculating a temperature
distribution in the plate-thickness direction by calculus of finite differences.
Reheating temperature after hot rolling 880°C or more
[0056] If the reheating temperature is lower than 880°C, target strength and toughness are
not achieved due to incomplete austenitization. Thus, the reheating temperature is
set to 880°C or more and is preferably set to 900°C or more. The upper limit temperature
for the reheating temperature is not particularly limited but is preferably set to
1000°C or less because heating to an excessively high temperature causes the size
of austenite grains to be increased, which leads to degradation of toughness.
Tempering temperature: 450°C to 680°C
[0057] If the tempering temperature is less than 450°C, the effect of tempering cannot be
produced to a sufficient degree. On the other hand, if tempering is performed at a
tempering temperature exceeding 680°C, a carbonitride having a large size is precipitated,
which unfavorably degrades toughness. When tempering is performed by induction-heating,
an increase in the size of carbide during tempering is favorably suppressed. In this
case, a temperature at the center of the thickness of a steel plate, which is calculated
by simulation such as calculus of finite differences, is set to 450°C to 680°C.
EXAMPLES
[0058] Slabs prepared from Steel Nos. A to N having the chemical compositions shown in Table
1 by continuous casting were used as raw materials, and hot rolling and a heat treatment
were performed under the conditions shown in Table 2. Thus, thick steel plates each
having a thickness of 60 to 150 mm were prepared.
[0059] A method for evaluating a base material was as follows. In a tensile test, a JIS
No. 4 test piece was taken from a 1/2 portion of a steel plate in its thickness direction
so that the longitudinal direction of the test piece was perpendicular to the roll
direction of the steel plate. Then, the yield point and tensile strength of the test
piece were measured.
[0060] In a Charpy impact test, a JIS V-notch test piece was taken from a 1/2 portion of
a steel plate in its thickness direction so that the longitudinal direction of the
test piece was perpendicular to the roll direction of the steel plate. Then, the absorption
energy at -40°C (vE-40°C) of the test piece was measured. The base-material properties
were evaluated as good when YP ≥ 620 MPa, TS ≥ 720 MPa, and vE-40°C ≥ 100 J were all
satisfied.
[0061] In the evaluation of welded zone toughness, a multipass welded joint was formed by
submerged arc welding at a welding heat input of 45 to 50 kJ/cm using a double bevel
groove. Absorption energy at -40°C was measured by setting a notch position for a
Charpy impact test at a weld bonded portion located on the straight-side of a 1/4
portion of a steel plate. The toughness of a welded zone joint was evaluated as good
when the average of three test pieces satisfied vE-40°C ≥ 100 J.
[0062] In addition, a CTOD value at -10°C was measured by setting a notch position for a
three-point bending CTOD test piece at a weld bonded portion located on the straight-side.
The CTOD characteristics of a welded joint was evaluated as good when the minimum
CTOD value among three test pieces was 0.50 mm or more.
[0063] Steels A to E and N were Invention Examples and Steels F to M were Comparative Examples
that did not meet the ranges of components specified in Claims. In Examples 1, 2,
5, 6, 10, 11, and 20, the components and manufacturing conditions according to the
present invention were satisfied, and good base-material properties and CTOD characteristics
were produced. Moreover, vE-40°C ≥ 100 J was satisfied.
[0064] On the other hand, in Example 3, in which air-cooling was performed subsequent to
reheating, target base-material strength was not produced since a cooling rate was
less than 0.3°C/s. In Example 4, target base-material strength and toughness were
not produced since the cooling-stop temperature exceeded 350°C. In Example 8, target
base-material strength and toughness were not produced since the heating temperature
was less than 880°C. In Example 9, target base-material strength and toughness were
not produced since the tempering temperature was less than 450°C. In Example 7, target
base-material toughness and the CTOD value of a welded zone were not produced since
the rolling reduction ratio was less than 2.
[0065] In Example 12, target base-material toughness was not produced since the amount of
C added was less than the lower limit specified in the present invention. In Example
14, a target CTOD value of a welded zone was not produced since the amount of Ni added
was less than the lower limit specified in the present invention.
[0066] In Examples 13, 15, 17, and 19, the amounts of C, Ceq, Mn, and P, respectively, exceeded
the upper limit specified in the present invention, and the value of HV max / HV ave
did not meet the range specified in the present invention. As a result, a target CTOD
value of a welded zone was not produced.
[0067] In Example 16, although the amount of each component fell within the range specified
in the present invention range, a center-segregation zone hardness index HCS = 5.5[C]
4/3 + 15[P] + 0.90[Mn] + 0.12[Ni] + 0.53[Mo] ≤ 2.5 was not satisfied. As a result, a
target weld portion CTOD value was not produced.
[0068] In Example 18, target base-material strength and toughness were not produced since
the amount of B added was less than the lower limit specified in the present invention.
[0069] A CTOD test and a Charpy test for a welded zone were not carried out in Examples
3, 4, 8, 9, 12, and 18, for target base-material strength and toughness were not scored.
[Table 1]
Table 1 |
(mass%) |
Steel |
C |
Si |
Mn |
P |
S |
Cu |
Ni |
Cr |
Mo |
V |
Ti |
B |
Sol.Al |
N |
Ca |
O |
Ceq |
HCS |
Remark |
A |
0.125 |
0.25 |
1.86 |
0.006 |
0.0004 |
0.02 |
1.26 |
0.03 |
0.02 |
0,001 |
|
0.0011 |
0.059 |
0.0040 |
|
0.0025 |
0.531 |
2.27 |
Invention Example |
B |
0.070 |
0.19 |
1.66 |
0.007 |
0.0007 |
0.02 |
1.89 |
0.75 |
0.45 |
0.041 |
0.009 |
0.0013 |
0.032 |
0.0030 |
0.0022 |
0.0024 |
0.722 |
2.22 |
Invention Example |
C |
0.082 |
0.12 |
0.95 |
0.007 |
0.0010 |
0.33 |
2.13 |
0.03 |
0.46 |
0.041 |
|
0.0012 |
0.059 |
0.0027 |
|
0.0033 |
0.511 |
1.66 |
Invention Example |
D |
0.113 |
0.25 |
1.45 |
0.007 |
0.0012 |
0.26 |
1.15 |
0.60 |
0.44 |
0.003 |
0.018 |
0.0013 |
0,053 |
0.0065 |
0.0025 |
0.0028 |
0.657 |
2.08 |
Invention Example |
E |
0.072 |
0.07 |
0.86 |
0.007 |
0.0023 |
0.01 |
0.95 |
2.20 |
0.12 |
0.002 |
0.022 |
0,0015 |
0.022 |
0,0035 |
0,0016 |
0.0037 |
0.744 |
1.22 |
Invention Example |
F |
0.040 |
0.28 |
0.88 |
0.006 |
0.0010 |
0.26 |
0.78 |
0.02 |
0.44 |
0.042 |
|
0.0012 |
0.053 |
0.0025 |
|
0.0022 |
0.356 |
1.28 |
Comparative Example |
G |
0.148 |
0.28 |
1.15 |
0.007 |
0.0008 |
0.01 |
0.89 |
0.70 |
0.65 |
0.002 |
0.015 |
0.0010 |
0.015 |
0.0055 |
|
0.0024 |
0.670 |
2.02 |
Comparative Example |
H |
0.121 |
0.25 |
1.24 |
0.004 |
0.0008 |
0.35 |
0.42 |
0.60 |
0.55 |
0,040 |
|
0,0014 |
0,017 |
0.0043 |
|
0.0029 |
0.617 |
1.85 |
Comparative Example |
I |
0.125 |
0.28 |
1.66 |
0.005 |
0.0025 |
0.03 |
1.46 |
0.86 |
0.68 |
0.058 |
|
0.0012 |
0.062 |
0.0027 |
|
0.0035 |
0.821 |
2.45 |
Comparative Example |
J |
0.096 |
0.25 |
2.25 |
0.004 |
0.0009 |
0.26 |
1.16 |
0.32 |
0,45 |
0.041 |
0.016 |
0.0012 |
0.016 |
0,0042 |
|
0.0037 |
0.728 |
2.70 |
Comparative Example |
K |
0.054 |
0.13 |
2.42 |
0.004 |
0.0006 |
0.22 |
0.55 |
0.21 |
0.01 |
0.041 |
0.016 |
0.0012 |
0,005 |
0.0042 |
|
0,0028 |
0,561 |
2.42 |
Comparative Example |
L |
0.057 |
0.13 |
1.58 |
0.004 |
0.0007 |
0.26 |
1.17 |
0.60 |
0.44 |
0.041 |
0.016 |
0.0001 |
0.015 |
0.0042 |
|
0.0036 |
0.632 |
1.98 |
Comparative Example |
M |
0,094 |
0.25 |
1.85 |
0.011 |
0.0029 |
0.04 |
0.66 |
0.83 |
0.52 |
0.055 |
|
0.0009 |
0.066 |
0.0032 |
|
0,0028 |
0.730 |
2.42 |
Comparative Example |
N |
0.059 |
0.23 |
1,05 |
0.003 |
0.0011 |
0.12 |
3.48 |
0.44 |
0.61 |
0.035 |
|
0.0015 |
0.067 |
0.0035 |
0.0021 |
0.0012 |
0.691 |
1.86 |
Invention Example |
Note 1: underlined part is out of the range of the present invention
Note 2: Ceq = C + Mn/6 + Cu/15 + Ni/15 + Cr/5 + Mo/5 + V/5 each element symbol represents
the content (mass%) of the element
Note 3: HCS = 5.5[C]4/3 + 15[P] + 0.90[Mn] + 0.12[Ni] + 0.53[Mo] [M] represents the content (mass%) of the
element, and the range of the present invention ≤ 2.5 |
[Table 2]
[0070]
Table 2
|
|
|
Rolling conditions |
Reheating-cooling conditions |
|
Base-material properties |
Welded portion toughness |
|
No |
Steel No. |
Steel slab thickness (mm) |
Heating temperature (°C) |
Steel plate thickness (mm) |
Rolling reduction ratio |
Temperature (°C) |
Cooling rate (°C/s) |
Cooling-stop temperature (°C) |
Tempering temperature (°C) |
YP (MPa) |
TS (MPa) |
vE-40°C (J) |
1.35 + 0.006/C - t/750 |
HV max /HV ave |
vE-40°C (J) |
CTOD δ-10°C (mm) |
Remark |
1 |
A |
250 |
1120 |
75 |
3.3 |
930 |
3 |
≤ 250 |
650 |
746 |
812 |
215 |
1.30 |
1.25 |
176 |
0.91 |
Invention Example |
2 |
B |
250 |
1120 |
100 |
2.5 |
930 |
2 |
≤ 250 |
610 |
822 |
884 |
198 |
1.30 |
1.24 |
189 |
0.76 |
Invention Example |
3 |
B |
250 |
1120 |
100 |
2.5 |
930 |
0.1 |
≤ 250 |
610 |
612 |
724 |
233 |
1.30 |
|
|
|
Comparative Example |
4 |
B |
250 |
1120 |
100 |
2.5 |
930 |
2 |
480 |
610 |
587 |
695 |
86 |
1.30 |
|
|
|
Comparative Example |
5 |
C |
250 |
1120 |
60 |
4.2 |
930 |
6 |
≤ 250 |
620 |
706 |
811 |
177 |
1.34 |
1.23 |
213 |
1.23 |
Invention Example |
6 |
D |
250 |
1120 |
100 |
2.5 |
930 |
2 |
≤ 250 |
650 |
774 |
865 |
189 |
1.27 |
1.25 |
167 |
0.87 |
Invention Example |
7 |
D |
250 |
1120 |
150 |
1.7 |
930 |
0.9 |
≤ 250 |
600 |
722 |
815 |
96 |
1.20 |
1.32 |
176 |
0.28 |
Comparative Example |
8 |
D |
250 |
1120 |
100 |
2.5 |
850 |
2 |
≤ 250 |
630 |
624 |
730 |
44 |
1.27 |
|
|
|
Comparative Example |
9 |
D |
250 |
1120 |
100 |
2.5 |
930 |
2 |
≤ 250 |
410 |
634 |
916 |
48 |
1.27 |
|
|
|
Comparative Example |
10 |
E |
300 |
1120 |
100 |
3.0 |
930 |
2 |
≤ 250 |
670 |
735 |
817 |
188 |
1.30 |
1.24 |
166 |
1.05 |
Invention Example |
11 |
E |
300 |
1120 |
130 |
2.3 |
930 |
0.9 |
≤ 250 |
630 |
723 |
805 |
198 |
1.26 |
1.21 |
168 |
0.79 |
Invention Example |
12 |
F |
250 |
1120 |
100 |
2.5 |
930 |
2 |
≤ 250 |
630 |
656 |
764 |
35 |
1.37 |
|
|
|
Comparative Example |
13 |
G |
250 |
1120 |
100 |
2.5 |
930 |
2 |
≤ 250 |
630 |
769 |
894 |
208 |
1.26 |
1.31 |
178 |
0.38 |
Comparative Example |
14 |
H |
250 |
1120 |
100 |
2.5 |
930 |
2 |
≤ 250 |
630 |
744 |
870 |
167 |
1.27 |
1.17 |
32 |
0.46 |
Comparative Example |
15 |
I |
250 |
1120 |
100 |
2.5 |
930 |
2 |
≤ 250 |
630 |
875 |
962 |
175 |
1.26 |
1.28 |
55 |
0.40 |
Comparative Example |
16 |
J |
250 |
1120 |
100 |
2.5 |
930 |
2 |
≤ 250 |
610 |
739 |
820 |
166 |
1.28 |
1.42 |
|
0.31 |
Comparative Example |
17 |
K |
250 |
1120 |
100 |
2.5 |
930 |
2 |
≤ 250 |
630 |
685 |
793 |
159 |
1.33 |
1.43 |
185 |
0.33 |
Comparative Example |
18 |
L |
250 |
1120 |
100 |
2.5 |
930 |
2 |
≤ 250 |
630 |
558 |
678 |
24 |
1.32 |
|
|
|
Comparative Example |
19 |
M |
250 |
1120 |
100 |
2.5 |
930 |
2 |
≤ 250 |
650 |
768 |
872 |
169 |
1.28 |
1.31 |
164 |
0.43 |
Comparative Example |
20 |
N |
300 |
1150 |
150 |
2.0 |
910 |
1 |
≤ 250 |
675 |
706 |
792 |
178 |
1.25 |
1.21 |
154 |
2.14 |
Invention Example |
Note 1: underlined part is out of the range of the present invention |
1. Hochfestes Stahlblech, das eine Fließgrenze von 620 MPa oder mehr hat und eine durch
Schweißwärme beeinflusste Zone mit Tieftemperaturzähigkeit von vE-40°C ≥ 100 J aufweist,
wobei das hochfeste Stahlblech eine chemische Zusammensetzung hat, die in Masseanteilen
besteht aus:
C: 0,05% bis 0,14%, Si:0,01% bis 0,30%, Mn:0,3% bis 2.3%, P:0,008% oder weniger, S:0,005%
oder weniger, Al:0,005% bis 0,1%, Ni:0,5% bis 4%, B:0,0003% bis 0,003%, N:0,001% bis
0,008%,
Ceq = [C] + [Mn]/6 + [Cu + Ni]/15 + [Cr + Mo + V]/5 ≤ 0,80, (wobei jedes Element-Symbol
den Gehalt (Masse-%) des Elementes repräsentiert),
wobei für einen Index der Härte einer Mittenseigerungs-Zone HCS Ausdruck (1) gilt,
und es wahlweise des Weiteren in Massenanteilen aus einem oder mehreren Element/en
besteht, das/die ausgewählt wird/werden aus:
Cr:0,2% bis 2.5%, Mo:0,1% bis 0,7%, V:0,005% bis 0,1%, Cu:0,49% oder weniger, Ti:0,005%
bis 0,025% und Ca:0,0005% bis 0,003%, und der Rest Fe und unvermeidbare Verunreinigungen
sind,
und für die Härte einer Mittenseigerungs-Zone in dem Stahlblech Ausdruck (2) gilt,
wobei jedes [M] den Gehalt (Masse-%) des Elementes repräsentiert,
wobei HVmax eine maximale Vickers-Härte der Mittenseigerungs-Zone repräsentiert, HVave eine durchschnittliche Vickers-Härte eines Abschnitts repräsentiert, der die Mittenseigerungs-Zone
nicht einschließt und der keine Bereiche einschließt, die sich von beiden Oberflächen
bis zu 1/4 der Dicke des Stahlblechs erstrecken, C den Gehalt (Masse-%) an Kohlenstoff
repräsentiert, und t eine Dicke (mm) des Stahlblechs repräsentiert,
wobei das Stahlblech eine Dicke von 60 bis 150 mm hat und einen CTOD-Wert einer durch
Schweißwärme beeinflussten Zone in einer Mehrlagenschweiß-Zone von 0,50 mm oder mehr
bei -10°C hat.
2. Verfahren zum Herstellen eines hochfesten Stahlblechs, das eine durch Schweißwärme
beeinflusste Zone mit Tieftemperaturzähigkeit aufweist, nach Anspruch 1, wobei das
Verfahren Erhitzen eines Stahls, der die chemische Zusammensetzung nach Anspruch 1
hat, auf 1050°C oder mehr und 1200°C oder weniger, Durchführen von Warmwalzen bei
einem Abwalzverhältnis (rolling reduction ratio) von 2 oder mehr, Durchführen von
Wiedererhitzen auf 880°C oder mehr und 1000°C oder weniger, Durchführen von Abkühlen
bei einer Abkühlgeschwindigkeit von 0,3°C/s oder mehr, bis eine Temperatur eines Mittelabschnitts
in einer Blech-Dickenrichtung 350°C oder weniger erreicht, und Durchführen einer Anlassbehandlung
bei 450°C bis 680°C.